Process for preparing plastic surfaces to be plated

Abstract: A process for sensitizing plastic surfaces prior to etching, which sensitization enhances the coverage and adhesiveness of the subsequent plate. The process of the invention uses limonene, solutions of limonene and emulsions of limonene as the sensitizer.

Description: This invention relates to a novel process used in preparing plastic surfaces so that the surface as treated can be effectively plated with an adherent coating. The plating of plastic parts is well known and of considerable commercial importance because the desirable characteristics of both the plastic and the metal are efficiently utilized in many applications. Thus a significant demand exists for metal plated plastic parts of all types. This includes not only plated plastic parts in general, but also the plated through-holes of printed circuit boards. In general, the process of plating plastics consists of etching the plastic surface followed by conditioning, activation and metallization. The problems in plating plastics are well known in the art. One of the most serious problems is providing a plate which is sufficiently adherent to the plastic surface. Another equally troublesome problem is providing for complete and even coverage of the plastic surface. Various methods have been employed in the past to rectify these problems. One approach is to provide for a plastic surface pre-etch sensitizer which will sensitize the plastic surface and thereby allow the subsequent plastic etch to more effectively etch the plastic surface. It has been demonstrated that through this approach of etching a sensitized plastic surface more effectively both improved coverage and adhesion can be achieved. It is thought that improved coverage is achieved in this way because the rougher plastic surface adsorbs more plating catalyst and thereby plates more efficiently. It is also thought that the increased adhesion of the plate to the surface is due to the increased roughness of the etched plastic surface. Various sensitizers are known in the art U.S. Patent No. 3,758,332 discloses the use of chemicals such as methyl ethyl ketone, tetrahydrofuran, dioxane, pyridine, dimethylformamide and various alcohol mixtures. U.S. Patent No. 4,125,649 reveals the use of halogenated alcohols, ketones and aldehydes as sensitizers for polysulfone resins. U.S. Patent No. 4,775,557 discloses the use of a compound of the general formula, R 1 (OC n H 2n ) m O-R 2 as a sensitizer for polycarbonate resins, wherein R 1 and R 2 are selected from aryl or alkyl groups containing from 1-4 carbon atoms, n is 2 to 4 and m is 1 to 5. Finally U.S. Patent Nos. 3,579,365 and 3,563,784 disclose an aqueous emulsion of turpentine with various surfactants as a sensitizing solution for acrylonitrile-butadiene-styrene (ABS). Various etchants for plastic which follow the sensitizer are known in the prior art including sulfuric acid, chromic acid, and permanganate solutions. These and other etch solutions are used following a sensitizer to etch and roughen the surfaces of various plastics in order to prepare these plastic surfaces for plating. These sensitizers of the prior art have achieved mixed results. They have generally been moderately effective at increasing the coverage and adhesion of plated metal deposits on plastic substrates. Many of the prior art sensitizers were directed to ABS type plastics and are completely ineffective in sensitizing the polycarbonates and polycarbonate alloys which are extensively used today. But the most significant drawback of these prior art sensitizers is that they are relatively dangerous to handle and environmentally unsafe. US 3533828 discloses a method of conditioning a solvent soluble plastic surface of an article so as to accept a firmly adhered metal plating. The method comprises contacting the surface with a treatment fluid comprising a solvent. The solvent may be dipentene. It has now been discovered that both the coverage and adhesion of plated metal deposits on plastic substrates, particularly polycarbonate and polycarbonate alloy substrates, can be enhanced by using a specific sensitizer solution prior to etching of the plastic surface. It has additionally been discovered that this particular sensitizer solution is both relatively safe to handle and environmentally friendly. The present invention provides a process as defined in claim 1. Limonene has either one or both of the following structures: Another common name which describes these indicated structures is dipentene, thus references to Limonene shall be taken to include either or both of the indicated structures and shall be interpreted as being synonymous with dipentene and with p-mentha-diene which is another common name describing structure 2. p-mentha-diene describes a group of compounds encompassing structure 2, but also includes structures the same as structure 2 but in which the ring double bond is shifted to any of the other 5 possible ring positions. Thus all p-mentha-dienes, Limonenes and dipentenes consisting essentially of the d-isomer can successfully be used in the practice of this invention. d-Limonene is the most prevalent form of Limonene in nature. d-Limonene occurs naturally in a wide variety of plants and is therefore readily available at low cost. It is derived from distilled orange rind oil and is therefore readily produced from citrus sources at low cost. It has been found that d-Limonene is also a very active plastic sensitizer. The sensitizer solution of this invention thus comprises Limonene consisting essentially of the d-limonene isomer, or more preferably an aqueous emulsion thereof comprising the Limonene, water and an emulsifier which is effective in forming a stable emulsion of Limonene in water. The composition of the invention is employed at an effective temperate which, depending upon the actual composition, may vary from below room temperature to the boiling point of the composition. Preferably, the composition is employed at a temperature of from 49 to 60°C [120 to 140°F]. The effective contact time will vary with the composition and temperature and preferably will be from 3 to 5 minutes. Following sensitization of the plastic, the plastic will be rinsed and then treated with an appropriate etchant. This etchant can be any effective plastic etchant including alkali metal hydroxides, chromic acid, sulfuric acid and permanganate solutions. The concept of the invention is the sensitization of the plastic with Limonene consisting essentially of d-limonene prior to etching which sensitization enhances the etchability of the plastic and therefore increases coverage and adhesion of the plated deposit to follow. Thus it has been found that Limonene consisting essentially of d-limonene and solutions and emulsions thereof achieve this result with a variety of etchants and metallization processes which follow the sensitization. After etching, a standard metallization cycle will include a neutralization step, an activation step and an acceleration step followed ultimately with metallization, there being appropriate rinse steps interposed between each step of the cycle. The time, temperature and compositions of each step following the sensitization may vary and are well known in the prior art. The process of the invention has been found to be particularly useful for sensitizing or conditioning plastics prior to plating and thus enhancing the adhesion and coverage of the plated deposit to and of the plastic surface. This process is useful in plating a variety of plastics including polysulfones, polypropylene, polyethylene, ABS and polyetherimides, but is particularly useful in plating polycarbonate and polycarbonate alloy resins. Although the sensitizer or conditioner of this invention may comprise Limonene consisting essentially of d-limonene or solutions thereof with various solvents, the preferred embodiment of this invention consists of an aqueous emulsion of Limonene consisting essentially of d-limonene and most preferably a stable, clear aqueous emulsion of Limonene consisting essentially of d-limonene. This most preferred embodiment of this invention comprises water, Limonene consisting essentially of d-limonene and an emulsifier or emulsifiers which are effective to form a stable, clear emulsion of the Limonene in water. The emulsifying agents which may be used in this most preferred embodiment vary widely. One exemplary emulsifier is Mazclean EP which is a proprietary non-ionic surfactant sold by PPG Industries. Another suitable emulsifier is Citrosperse 120 which is a proprietary surfactant blend available from ICI Corporation. Finally a combination of Triton X-100, a proprietary surfactant sold by Rhom and Haas, and Dowfax 2A1 sold by Dow Chemicals, is also an effective emulsifier of Limonene in water. The process to be used in employing the sensitizer of this invention will consist of the following steps; 1. Sensitization 2. Etch 3. Neutralization 4. Activation 5. Acceleration 6. Metallization along with appropriate rinse steps interspersed between each process step. The sensitization step will comprise either Limonene consisting essentially of d-limonene, a solution thereof in an appropriate solvent or an emulsion thereof, preferably in water. This will be carried out by immersion or spray at a temperature ranging from room temperature to the boiling point of the composition, but preferably from between 49 to 60°C [120 and 140°F]. The contact time will vary with the composition, temperature and the particular plastic being processed, but preferably will be from 3 to 5 minutes. The etch step will consist of a variety of plastic etchants including without limitation alkali metal hydroxides, sulfuric acid, chromic acid, and permanganate solutions. Times and temperatures vary depending upon the composition and plastic under consideration. Neutralization will effectively neutralize any etchant residues on the surface of the plastic. The particular neutralizer used depends upon the etchant used. For example, a suitable neutralizer for a permanganate etch is an acidic solution of hydrogen peroxide. Activation consists of preparing the surface of the plastic so that it is catalytic to subsequent plating. The composition of this step can vary somewhat depending upon the particular type of plating which will follow. In the preferred embodiment, activation will be achieved with a precious metal catalyst, most preferably, a tin-palladium colloidal solution. The following acceleration step is optional with the purpose being enhancement of the activation of the surface. The final step of metallization may in fact consist of several steps itself. In the preferred embodiment, the first step of metallization will consist of an electroless plating bath which is capable of depositing a metal layer on the activated surface of the plastic, the most preferred electroless plating bath being an electroless copper plating bath. The use of Limonene consisting essentially of d-limonene to sensitize plastic surfaces before etching has a variety of advantages. The most important advantages of using Limonene consisting essentially of d-limonene as a pre-etch plastic sensitizer is that it increases both the adhesion of the subsequent plated deposit to the plastic surface and the coverage of the plate over the surface. These characteristics are obviously extremely important in the aesthetics and functioning of plated plastic parts. In addition, Limonene consisting essentially of d-limonene and solutions and emulsions thereof are relatively safe to handle and environmentally friendly. Limonene for the most part is derived from natural citrus sources and therefore provides these important safety and environmental advantages over the synthetic, inherently more dangerous and unsafe sensitizers of the prior art. The invention is further described in the following examples which are given to further illustrate the invention but shall be in no way limiting. The following standard process sequence was used to plate a variety of plastic parts: a) Sensitize, 3-5 minutes, 49-60°C [120-140°F] b) Rinse, minutes, room temperature c) Etch, 7 minutes, 74°C [165°F] (MacDermid L-65 chromic acid etch was used) d) Rinse, 3 min., room temperature e) Neutralize, 2 min., 43°C [110°F] (MacDermid 9339 neutralizer was used) f) Rinse, 3 min., room temperature g) Activate, 3 min., 29°C [85°F] (MacDermid D-34C activator was used) h) Rinse, 3 min., room temperature i) Accelerate, 2 min., 49°C [120°F] (MacDermid MAC 25 Accelerator was used) j) Rinse, 3 min., room temperature k) Electroless Copper, 15 min., 43°C [110°F] (MacDermid 7960 electroless copper was used) l) Rinse, 3 min., room temperature m) Activate, 2 min., 49°C [120°F] (MacDermid D-45 Activator was used) n) Rinse, 3 min., room temperature o) Electroless Nickel, 3 min., 60°C [140°F] (MacDermid 813 electroless nickel used) p) Rinse, 3 min., room temperature q) Dry This entire process was held constant through all examples for comparison purposes. The only step which varied was the sensitization step. Example I The sensitizer used had the following composition: d-Limonene 12.96% by wt. Butyl Carbitol 56.18% by wt. Water 30.86% by wt. The following plastic materials were run through this sensitizer solution and the standard cycle. The coverage and adhesion results were recorded. Coverage was gauged by the % area covered by plate. Adhesion was measured using the ASTM D-3359-87 standard cross hatch test for determining adhesion of a plate to a substrate and was graded on a scale of 0 to 5 with 5 being the most superior adhesion. Coverage (%) Adhesion FR-90 Polycarbonate/ABS Alloy 100 4 GE-8002 Polycarbonate/ABS Alloy 100 5 FL-900 Polycarbonate 100 3 Example II The sensitizer used had the following composition: d-Limonene 19.2% by wt. Ethylene Glycol 23.7% by wt. Mazclean EP 1 23.0% by wt. Water 34.1% by wt. The following plastic materials were run through this sensitizer and the standard cycle. The coverage and adhesion values were determined and recorded as in Example I. Coverage (%) Adhesion FR-1441 Polycarbonate/ABS Alloy 100 4 GE-2800 Polycarbonate/ABS Alloy 100 4 T-85 Polycarbonate/ABS Alloy 100 4 1. Available from PPG Industries Example III The sensitizer used had the following composition: d-Limonene 21% by wt. Mazclean EP 23.4% by wt. Butyl Carbitol 5.6% by wt. Water 50% by wt. Plastic materials were processed through this sensitizer and the standard cycle. The coverage and adhesion values were determined and recorded as in Example I. Coverage (%) Adhesion FL-900 Polycarbonate 100 5 GE-8002 Polycarbonate/ABS Alloy 100 5 Pulse 1370 Polycarbonate/ABS Alloy 100 5 FR-90 Polycarbonate/ABS Alloy 100 5 Example IV The sensitizer used had the following composition: d-Limonene 33.3% by wt. Citrosperse 120 1 16.7% by wt. Water 50% by wt. Plastic materials were processed through this sensitizer and the standard cycle. The coverage and adhesion values were determined and recorded as in Example I. Coverage (%) Adhesion GE-2800 Polycarbonate 100 5 GE-8007 Polycarbonate/ABS Alloy 100 5 FR-90 Polycarbonate/ABS Alloy 100 5 T-85 Polycarbonate/ABS Alloy 100 5 1. Available from ICI Corporation Example V The following sensitizer composition was used: d-Limonene 21% by wt. Mazclean EP 23.4% by wt. Butyl Carbitol 5.6% by wt. Water 50% by wt. Plastic materials were processed through this sensitizer and the standard cycle. The coverage and adhesion values were determined and recorded as in Example I. Coverage (%) Adhesion Dow Styron 6087 Polystyrene 100 4 GE Prevex VJB Polyphenylene Ether 100 4 Thus sensitizers comprising limonene consisting essentially of d-limonene are effective in enhancing the adhesion and coverage of a variety of plastic substrates. In addition, these sensitizers are relatively safe to handle and safe for the environment.

Claims: A process for treating a plastics material prior to etching a surface thereof so as to enhance the coverage and adhesiveness of subsequent plating on the etched plastics surface, which process comprises contacting the plastics material for an effective time and at an effective temperature with a sensitizer composition comprising dipentene as the pre-etching component characterized in that the dipentene consists essentially of d-limonene isomer. A process according to Claim 1, wherein the sensitizer composition comprises a solution of d-limonene in an appropriate solvent. A process according to Claim 1, wherein the sensitizer composition comprises: a) d-limonene, b) an emulsifier effective to form an aqueous emulsion with component (a), and c) water. A process according to any one of the preceding claims, wherein the plastics material is selected from polycarbonates, polycarbonate alloys, polyetherimides, polysulfones, polyphenylene ethers, polypropylenes and polyethylenes. A process according to Claim 4, wherein the plastics material is a polycarbonate or a polycarbonate alloy.

Inventors: BOEHM DENNIS R, DIMARCO MASSIMO, KUZMIK JOHN J, MORRIS HOWARD L, BOEHM, DENNIS R,, DIMARCO, MASSIMO, KUZMIK, JOHN J., MORRIS, HOWARD L.
Applicants: MACDERMID INC, MACDERMID INCORPORATED
Application year: 1994
Main classification: C23C18
IPC codes: C23C18, H05K3, B05D3, C08J7